Asked if Russia could retaliate to the US move to supply weapons to the Syrian rebels by delivering the S-300 air defence missile systems to the regime, Ushakov said "there is no talk about it yet".

"We aren't competing over Syria, we are trying to settle the issue in a constructive way," he said.

On Friday, UN Secretary-General Ban Ki-moon said that "the validity of any information on the allege use of chemical weapons cannot be ensured without convincing evidence of the chain-of-custody", and said increasing the flow of arms to either side "would not be helpful".

'Chemical weapons used'

The US decision and announcement come as Assad's forces have been scoring a series of victories, driving rebels out of a key town near the Lebanese border.

They have also launched offensives in the central and northern parts of the country. Aleppo, the country's largest city, has been particularly targeted.

However, no final decisions have been made on the type of weaponry or when it would reach the rebels, according to the officials, who spoke on condition of anonymity in order to discuss internal administration discussions with reporters.US officials said the administration could provide the rebel fighters with a range of weapons, including small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired rocket-propelled grenades and other missiles.

In addition to the military aid, the US also announced on Thursday that it had conclusive evidence that Assad's regime had used chemical weapons, including the nerve agent sarin, on a small scale against opposition forces. The White House said multiple chemical attacks last year killed up to 150 people.

Obama had earlier said that the use of chemical weapons cross a "red line", triggering greater US involvement in the crisis.

'Blood of Syrians'

The Syrian opposition has said that representatives from the Free Syrian Army (FSA), an umbrella group of armed opposition fighters, are to begin meeting with foreign officials from Saturday to discuss what form military aid could take.

"We hope to have the weapons and ammunition that we need in the near future," General Salim Idris, the commander of the FSA, said.

"We encourage them to take a decision in this relation, by establishing a no-fly zone either all over Syria or areas they choose based on their technical or military considerations on the ground" to ensure safe areas for civilians, Loay Mikdad, an FSA spokesman, said.

"We hope they start arming immediately. Any delay costs blood of Syrians. It is not water, it is blood of the Syrians, women and children and its future."